Kewaunee Explosively Expands Tubes to Fit New Sleeves
By James T. VanRemortel,Framatome Technologies Inc.
In September 1996, Wisconsin Public Service Corp. (WPS) Took Kewaunee Nuclear Power Station
off-line for a scheduled refueling and maintenance outage. During the outage at the 23-year-old, 563 MW Westinghouse pressurized water reactor plant, engineers performed routine eddy current inspections of the steam generator tubes. The eddy current analyses revealed a large number of defects in the tube regions where hybrid expansion joint (HEJ) sleeves had been installed in the late 1980s. The defects were located in the parent tube, in the hard-roll region of the upper joint of the sleeve.
The steam generator maintenance vendor attempted to repair the HEJ sleeves using a laser-weld technique to establish a new code pressure boundary, but was unsuccessful. After several months of iterations, it became apparent that the majority of the sleeved tubes had defects requiring their removal from service. Because the resulting reduction in the heat-transfer area in the steam generators would have prevented plant operation under its current license, WPS sought an innovative repair for the defective sleeves.
Choosing a Method
WPS asked Framatome Technologies Inc. (FTI) to remove several tube samples with defective HEJ sleeve-to-tube joint welds for analysis. After reviewing the sample geometry, FTI and teaming partner ABB-CE proposed a first-of-a-kind repair that required removing the majority of the existing sleeve and expanding the sleeve remnant to facilitate the passage of a new sleeve beyond the damaged area. The technology existed to remove the sleeve–except for the upper sleeve joint, which was expanded and, in some cases, welded to the parent tube. The challenge lay in devising a method for expanding the sleeve remnant and the parent tube to allow the passage of the ABB-CE sleeve. The process had to be performed without over-expanding the parent tube, which could damage adjacent tubes. To meet WPS` outage schedule requirements, the new method had to be developed, tested and qualified in approximately six weeks, including approval by Kewaunee Plant Engineering and, ultimately, approval by the Nuclear Regulatory Commission (NRC).
During the research phase, engineers tested several concepts. The first approach–roll expanding the sleeve segment and parent tube to an acceptable diameter–met with limited success. In some cases, the sleeve segment within the parent tube developed through-wall vertical cracks caused by cold working/strain hardening. The process was also very time consuming and did not always fully expand the upper welded joint region.
The second approach involved hydraulic expansion of a pressurized bladder, which would cause the sleeve remnant and parent tube to yield. This technique also experienced difficulties. The presence of locking probes on both ends of the sleeve remnant–necessary to permit bladder expansion–prevented the ends of the sleeve remnant from expanding. After bladder removal, the sleeve remnant ends still had to be roll expanded prior to new sleeve installation. The project team tested several variations of the roll expansion and localized pressure methods; inconsistent and poor-quality results, however, led to the conclusion that neither option was viable.
Engineers considered kinetic expansion as a third option. In this method, a controlled explosion is used to manipulate the tube/sleeve inside diameter. Kinetic expansion provides a rapid, precise way to expand all of the targeted tubes. After further evaluation by FTI Engineering and Kewaunee Engineering, WPS accepted the kinetic method as the expansion process of choice for the steam generator re-sleeving.
FTI began designing a shaped kinetic expansion device (KED) that, when detonated, would expand the region of interest to a precise diameter, while keeping the parent tube intact. The KED material needed uniform expansion properties and its chemical makeup had to be such that an environment conducive to corrosion would not be created in the steam generator if any residue remained after detonation and cleaning. Design engineers selected a polyurethane material for the delivery package because it met these criteria. FTI fabricated 16 mock-up tube samples with HEJ sleeves to simulate the original installation techniques. After finalizing the charge design, the project team detonated the KEDs in the qualification test samples. Results indicated that the expanded region of the specimens had the dimensions necessary to accommodate the ABB-CE sleeve.
The project team`s next hurdle involved ensuring the tube could be adequately cleaned and prepped for the new sleeve. Test results from various cleaning techniques led FTI and ABB-CE to believe that simply honing and brushing the location after expanding the tube would be adequate to remove the remains of the KED deposits. The inner surface of the parent tube would then be in an acceptable condition for the weld bond with the new sleeve.
Team members briefed the NRC on the expansion and cleaning processes to facilitate licensing and to prove that the methods were technically sound. Throughout the development process, continual updates to the NRC gained WPS prompt licensing approval.
To meet the aggressive turnaround schedule, FTI and ABB-CE performed a 25-tube demonstration in the “A” steam generator at Kewaunee. The figure illustrates the procedure used for the sleeve removal/new sleeve installation process. The first step of the removal process involved stress relief of the lower rolled joint region of the sleeve using a tungsten inert gas (TIG) process. Introducing heat to the rolled region relaxed the sleeve and freed it from the parent tube.
The next step focused on removing as much of the existing HEJ sleeve as possible. A specially designed, flexible cutting device–inserted through the sleeve end to an elevation just below the upper hydraulic expansion sleeve joint–severed the sleeve below the expanded region, releasing it from the upper weld joint.
The final step of the removal process involved gripping the inside of the sleeve segment, applying a load using a hydraulic jack and removing the sleeve segment from the parent tube.
The project team completed kinetic expansion of the 25 tube samples in 3 hours. The sample tubes were then free-pathed from the cold leg of the steam generator using a standard eddy current probe. The free-pathing step provided a mechanism to clean the larger pieces of debris from the tube as well as to obtain a base eddy current reading of the expanded tube. High-pressure, high-volume air was used to remove smaller pieces of debris from the expanded tubes.
To verify the diameter of the sleeve remnant section, engineers closely examined each tube`s expanded region (approximately six inches) with a profilometry probe. All profiles were verified to ensure sufficient tube diameters and to confirm that no over-expansions had occurred. This review determined that all 25 tubes were acceptable for re-sleeving.
As noted above, removal of oxidation and KED residues from the tube inside diameter is crucial. To ensure an acceptable condition for welding the new sleeve, the project team used a combination of hones and brushes in the area where the new TIG weld would be made. As an added precaution, prior to installing the new sleeves, technicians severed the parent tubes within the tubesheet region to alleviate stresses that may have developed from previous repairs or during the expansion process.
The new sleeves were installed extending from the tube end several inches beyond the original HEJ sleeve, thus covering both the original parent-tube defect and the old expanded sleeve remnant. During sleeve insertions, the top portions of the ABB-CE sleeves were hydraulically expanded into the parent tube and then TIG welded. The lower sleeve sections were roll expanded into the parent tube near the primary face, making a secure lower-sleeve joint. Technicians inspected the upper-sleeve welds using ultrasonic and eddy current testing to ensure that the welded joint regions met licensing requirements. The inspection showed that only 17 of the 25 newly installed sleeves were acceptable for service. Even though the entire process worked, it demanded improvements to increase the acceptance ratio.
Destructive examination of several specimens and a repaired tube removed from the “A” steam generator confirmed that the area of the upper joint was not adequately cleaned and prepared for the TIG-weld process. When an additional cleaning step for KED residue removal using a centrifugal scraper proved successful, WPS gave the go-ahead for the full-scale production batch of 361 tubes.
In total, FTI and ABB-CE installed 386 sleeves in the “A” steam generator at Kewaunee (WPS decided to concentrate the repair efforts on one steam generator to make the June restart schedule). Of the re-sleeved tubes, 20 had to be plugged based on eddy current indications. The greatest percentage of plugged sleeves, however, resulted from the initial 25-tube demonstration. Overall, the re-sleeving achieved a success rate of better than 94 percent, while meeting the desired schedule. The unit also experienced a smooth subsequent restart.
Kewaunee remains slightly power-limited due to the number of steam generator tubes still plugged. Nonetheless, the re-sleeving technique developed and implemented for the project was a success; it can be used to return to service certain categories of tubes currently plugged and to recover plugging margin/lost power. p
A 1996 outage at the 563 MW Kewaunee Nuclear Power Station provided an opportunity to utilize a kinetic expansion technique for re-sleeving steam generator tubes. Photo courtesy of Framatome Technologies Inc.
Author–James T. Van-Remortel is a project engineer with Framatome Technologies Inc. He has been with Framatome since 1993, serving as a field service engineer and as a robotics systems engineer. VanRemortel received his bachelor of science degree in electrical engineering from Lake Superior State University.